I have no evidence or real knowledge of this, so pay attention at your own risk, but I would try checking out Wegman's down by Quakerbridge, or maybe Macaffery's in Princeton Shopping Center as the 2 "high-end" supermarkets in the area.

If no luck at either of them, a ride up 27 to the Highland Park area, with its Jewish population might turn up a supermarket or deli that carries it.

I'm sure you've already checked here, but I know I've seen a variety of Dr. Brown's products at the Wegman's in Princeton. They're in the International section in the back of the store, in the first aisle.

There was a thread about Dr. Brown's Cel-Ray a few months back on the soft drink board of eGullet where some claimed it had been discontinued. I called around (it's owned by Canada Dry) and while it is still made, most stores in Central Jersey that carry Doctor Brown's other flavors DON'T stock Cel-Ray.

I've had the most luck at the Wegman's in Manalapan (tho' that store is often frustratingly out of their own store brand of Cranberry soda and even when they do have it, it's only in 2 liter bottles rather than 12 oz cans). Cranberry soda is even more rare in NJ than Cel-Ray, at least for me.